The present invention relates in general to optical equipment and, in particular, to a new and useful optical multiplexer, of splitting the light beam conducted in one light waveguide into at least two further light waveguides.
Multiplexers of this kind are known for example, from the articles: "Interference Filter All-Fibre Directional Coupler for W.D.M.", by H. F. Mahlein et al., Electronic Letters 16, 1980, pages 584 to 585, and "Compact Wavelength Multiplexer Using Optical-Fiber Pieces", by E. Miyanchi et al., Optics Letter 5, 1980 pages 321-322.
In such arrangements, the end faces of optical fiber pieces are disposed as close to each other as possible, in such a way as to permit a distribution of the light beam from one fiber to a plurality of fibers through partly transmitting or wavelength-dependent reflecting layers which are applied to polished oblique end faces of the fibers. In any of these prior art arrangements, a material is provided between the fiber ends, intended to match the indices of refraction of the fibers. Since all the mentioned arrangements employ dielectric reflecting layers, it is difficult or even impossible to heat-seal the fiber ends and further optical component parts to each other. A simple, conventional method of matching the indices is to fill the space between the fiber ends with a transparent plastic, for example, epoxy resin.
This is decidely a solution for experimental purposes; however, it is questionable whether such an arrangement has a satisfactory long-term stability required for industrial application. That is, the light intensities usually ocurring in fiber-optical systems are very high. for a gradient fiber having a core diameter of 50 microns and a luminous flux of 2 mW which is conducted in the fiber core, an intensity of 100 W per cm.sup.2 may be computed. This is a very high value. Therefore, the question arises whether, under a constant irradiance with a light of such intensity, an organic cement (such as epoxy resin) does not succumb to aging, which process may lead, for example, to a clouding of, or absorption in, the initially transparent cement.